1. Field of the Invention
The invention relates generally to the field of electromagnetic surveying. More specifically, the invention relates to structures for electrodes used in streamers forming part of a marine electromagnetic survey system.
2. Background Art
Electromagnetic geophysical surveying of the Earth's subsurface includes “controlled source” and “natural source” electromagnetic surveying. Controlled source electromagnetic surveying includes imparting an electric field or a magnetic field into subsurface Earth formations, such formations being below the sea floor in marine surveys, and measuring electric field amplitude and/or amplitude of magnetic fields by measuring voltage differences induced in electrodes, antennas and/or interrogating magnetometers disposed at the Earth's surface, or on or above the sea floor. The electric and/or magnetic fields are induced in response to the electric field and/or magnetic field imparted into the Earth's subsurface, and inferences about the spatial distribution of conductivity of the Earth's subsurface are made from recordings of the induced electric and/or magnetic fields.
Natural source electromagnetic surveying includes deploying multi-component ocean bottom receiver stations and by taking the ratio of perpendicular field components, one can eliminate the need to know characteristics of the natural source.
Controlled source electromagnetic surveying known in the art includes imparting alternating electric current into the subsurface formations. The alternating current has one or more selected frequencies. Such surveying is known as frequency domain controlled source electromagnetic (f-CSEM) surveying. Another technique for electromagnetic surveying of subsurface Earth formations known in the art is transient controlled source electromagnetic surveying (t-CSEM). In t-CSEM, electric current is imparted into the Earth at the Earth's surface (or sea floor), in a manner similar to f-CSEM. The electric current may be direct current (DC). At a selected time, the electric current is switched off, switched on, or has its polarity changed, and induced voltages and/or magnetic fields are measured, typically with respect to time over a selected time interval, at the Earth's surface or water surface. Alternative switching techniques are possible. Structure of the subsurface is inferred by the time distribution of the induced voltages and/or magnetic fields. For example, U.S. Patent Application Publication No. 2004/232917 and U.S. Pat. No. 6,914,433 Detection of subsurface resistivity contrasts with application to location of fluids (Wright et al.) describe a method of mapping subsurface resistivity contrasts by making multichannel transient electromagnetic (MTEM) measurements on or near the Earth's surface using at least one source, receiving means for measuring the system response and at least one receiver for measuring the resultant earth response. All signals from each source-receiver pair are processed to recover the corresponding electromagnetic impulse response of the earth and such impulse responses, or any transformation of such impulse responses, are displayed to create a subsurface representation of resistivity contrasts. The system and method enable subsurface fluid deposits to be located and identified and the movement of such fluids to be monitored.
The above methods for f-CSEM and t-CSEM have been adapted for use in marine environments. Cable based sensors have been devised for detecting electric and/or magnetic field signals resulting from imparting electric and/or magnetic fields into formations below the bottom of a body of water. See, for example, U.S. Patent Application Publication No. 2006/0238200 filed by Johnstad. The amplitude of the signals detected by electrodes on cables resulting from the imparted electric field, such as described in the Johnstad publication, may be on the order of fractions of a nanovolt. Accordingly, a particular consideration in the design and implementation of electromagnetic survey receiver systems is reducing the amount of noise that may be induced in the signals detected by the various sensing elements in the receiver system. An important part of reducing noise in such systems is careful attention to the design of electrodes used to measure voltages related to the amplitude of electromagnetic fields.
There is a need for improved voltage measuring electrodes used with electromagnetic survey systems.